1. Field of the Invention
The present invention relates generally to feedthroughs, and more particularly to, forming an electrically insulative structure having holes for feedthroughs.
2. Related Art
As used herein, a ‘feedthrough’ is an electrically conductive path extending through an insulative member, and which has portions accessible at each side of the insulative member. The electrically conductive path may extend from the interior of a hermetically sealed container or housing on one side of the insulative member, to an external location outside the container or housing on the other side of the insulative member. Typically, a conductive path is provided by an electrically conductive pin or rod, which is electrically insulated from the container or housing by an electrically insulating body surrounding the pin. As such, a feedthrough allows one or more electrical connections to be made between electronic circuitry or other components within a hermetically sealed container or housing and components outside the housing, while protecting the circuitry or components from any damage or malfunction that may result from exposure to the surrounding environment. A structure comprising a collection of one or more feedthroughs is sometimes referred to herein as a ‘feedthrough device.’
There are many applications for feedthrough devices. One exemplary application is in electrical devices for implantation in a patient's body to provide therapy to the patient, such as cardiac pacemakers, defibrillators and cochlear implants, collectively and generally referred to herein as implantable medical devices. As the environment of living tissue and body fluids is relatively corrosive and devices may contain materials which may be detrimental if exposed to the patient, a hermetic feedthrough device is used to provide a barrier between the electronic components of the medical device and the external corrosive environment of the human body. With implantable medical devices in particular, it is beneficial that the hermetic seal of the device be physically rugged and long lasting. For this reason, stringent requirements are imposed on the hermeticity of an implanted device, typically requiring a seal that provides a leakage rate of approximately less than 10−8 cc/sec.
As such, conventional feedthrough devices used in implantable medical devices typically consist of a ceramic or glass bead that is bonded chemically at its perimeter through brazing or the use of oxides, and/or mechanically bonded through compression, to the walls of the sealed package. A suitable wire or other conductor passes through the center of the bead, and this wire or conductor must also be sealed to the bead through chemical bonds and/or mechanical compression. Such feedthroughs are typically cylindrical and the wire(s) or conductor(s) mounted within the bead are centered or mounted in a uniform pattern, centrally within the bead. Other materials and processes are known for making conventional feedthroughs for implantable medical devices rely, for example, on use of aluminum oxide ceramic and binders.
One of the conventional processes for making a feedthrough consists of pre-drilling holes in a sintered ceramic plate and then forcing electrical conductive pins through the holes. Examples of such processes are disclosed in U.S. Pat. No. 5,046,242. While useful, this method is tedious, slow, does not necessarily guarantee a hermetic seal, and generally results in unsatisfactory leakage rates and yields. Furthermore, it has been found that drill bits wear quickly when used on ceramics due to the abrasive nature of the ceramics. Thus, to meet required tolerances, drill bits typically need to be replaced often. Also, the build-up of stress around punched or drilled holes can result in subsequent cracking of the sintered ceramic.
Another conventional method for making a feedthrough involves inserting the conductive pins into an unsintered ceramic plate then curing the assembly by firing to achieve a hermetic seal. A major disadvantage of this process is that, historically such processes were required to be performed by hand. Such a manual method of manufacture can lead to inaccuracies and may be time consuming, expensive and labor intensive. Moreover, feedthroughs resulting from such a process do not necessarily have precisely positioned electrical conductors, with the position of the conductors being greatly dependent upon the process itself. Furthermore, as the conductors are typically wires being of a general cylindrical shape and configuration, the size and shape of the accessible portions of the conductor are generally the same as the conductor embedded in the insulative material.
As implantable medical devices continue to develop and become thinner, smaller and more electronically sophisticated, the requirements of the feedthrough have also increased. For example, in certain cochlear implants, where there are 22-24 electrodes, there may be a need for 22-24 conductive pins passing through the feedthrough device. As the desire for more electrodes and smaller feedthrough devices increases, the demands placed upon the design of the traditional feedthrough also increases. The problems in fabricating feedthrough devices on a large scale are significant, especially when one considers the relatively high degree of labor intensity and specialization of current fabricating methods.
While the above described conventional feedthrough devices and fabrication methods have proven successful, it is a relatively slow and labor intensive process to manufacture such devices. These methods of manufacture of the feedthrough devices also presents limitations as to the construction of the feedthrough devices.
US Patent Publication No. 2006/0141861, by the present applicant, discloses various embodiments of methods for forming a feedthrough device. In the embodiment illustrated in FIG. 26 of this US publication, an electrically insulating structure having holes for feedthroughs is formed by powder injection molding (PIM). The mold includes a pair of opposed mold plates, with one of the plates carrying a number of pins and the other plate having recesses which receive the pins. The plates are slowly moved apart to expose a partial cavity, into which hot feedstock is injected. The feedstock is injected in the partial cavity around the exposed portions of the pins. The process of moving the plates apart and injecting feedstock into the further exposed cavity continues until the cavity is fully molded. Once this process is completed, the molded structure is ejected, the molded structure having holes formed therethrough where the pins were located. The holes then allow feedthrough conductors to be arranged through the molded structure. The content of US Patent Publication No. 2006/0141861 is hereby incorporated by reference herein.